Abstract: An airborne radio-telephone transceiver is provided having improved channeling and decode circuitry compatible with the selective signalling call-up system prescribed by the Federal Communications Commission. The decode circuitry includes a filter and detector dependent only on reception of a single selected frequency transmitted as a call-up signal by a ground station. A retriggerable, resettable, monostable multivibrator, configured as a pulse swallower, provides the necessary counter offset for proper call-up signal decoding. The invention includes scanning circuitry for rapid operator selection of active transmit-receive channels in the operator's geographic area.

Abstract: A frequency synthesizer employing direct FM, as opposed to phase modulation, so subaudible tones may be transmitted, in the form of a 10 KHz increment, direct synthesizer with a skewed frequency standard deriving four intermediate 2.5 KHz tuning steps. Fast settling time of frequency change accommodates semi duplex operation, and a shaping network provides the amount of skewing changes required between band ends for substantially perfect tracking. A low division ratio programmed divider in the system maximizes transmitter signal to noise ratio, and a low pass filter in the synthesizer phase locked loop allows voice band moduation.